The Evolution of the Universe 13 



Sandage (1958) gives a calculation of probably 13 to 20 billion 

 years. 



According to Sandage (1956), certain spectral shifts of distant 

 galaxies may indicate that the expansion of the universe is slowing 

 down. If this is true, it would suggest that the universe would 

 eventually expand to a certain limit, then contract again into a 

 titanic mass of neutrons and energy and thus start a new explosion. 

 As a result, the universe as a whole would have a cyclical nature. 

 A verification of the slowing down process would thus support the 

 explosion rather than the steady state theory. 



The steadtj state theory of the universe proposed by Hoyle, Bondi, 

 and Gold postulates that new hydrogen is being formed constantly 

 in inter-galactic space, that this new gas forms new great gas balls, 

 and that these evolve into galaxies. The production of new galaxies 

 theoretically equals the old ones which disappear over our horizon 

 of space. According to this explanation, the universe would always 

 look the same; it would have had no beginning and would continue 

 for an infinite time. 



EVOLUTION OF THE STARS AND ELEMENTS 



The explosion theory accounts satisfactorily only for the theoretical 

 production of hydrogen, the heavy isotopes of hydrogen ( deuterium 

 and tritium), and helium. The steady state theory postulates (but 

 gives no theoretical basis for) the original production of hydrogen. 

 In spite of their relative rarity on the earth, it is a fact that hydrogen 

 ( 76 per cent ) and helium ( 23 per cent ) do comprise the great bulk 

 of the matter in the entire universe (Fowler, 1956). However, in 

 spite of their relatively low abundance (together, only 1 per cent 

 of the weight of the universe) the other 90 chemical elements do 

 occur and play important roles in the evolutionary processes of at 

 least some parts of the solar system. Where these other elements 

 ( and possibly helium also ) originated was for some years a mystery, 

 but more recent studies show with little doubt that they had their 

 origin in the dynamic processes occurring in stars. Lucid accounts 

 of this entire topic are given by Fowler ( 1956 ) and the Burbidges 

 (1958). A brief outline of their ideas follows. 



If a portion of the cold, turbulent gas cloud of a galaxy that con- 

 tains only hydrogen consolidates to form a star, the gravitation 

 of the consolidating mass will compress the gas and, as a result, the 

 star's interior will become denser and hotter. When the central 



